Research during the past several years has focused upon DNA as the target primarily responsible for the loss of cellular proliferative capacity. With the renewed interest in neutrons for radiotherapy, information about the effects of neutrons at the molecular level is needed. The proposed work will contrast differences between the production of single and double strand DNA breaks after X-ray and neutron irradiation. Parallel measurements of cell survival will be made for purposes of correlations which are needed. Experiments will be performed under hypoxia as well as normal oxygenation, for estimation of O.E.R. values. Studies of DNA repair and cell survival will be conducted for synchronized cells irradiated with x-rays and neutrons. These measurements will provide information about the influence of the various portions of the cell cycle upon the response to neutron irradiation. Initial experiments will be started using pharmacologic agents in combination with X-rays and neutrons. These experiments will include measurements of DNA repair together with cell survival measurements. The data provided by this project will yield valuable information about molecular and cellular effects of fast neutrons. The energy spectrum of the neutron beam and dose rate represent an excellent model for purposes of planning for therapy.